A solution to this can be found on the iplayerv2 forums (http://code.google.com/p/xbmc-iplayerv2/issues/detail?id=7#c18), but no posts contain the exact error message, so i'm posting the solution here.
If you find that your XBMC IPlayer has suddenly stopped working with 'Can't locate a tv stream' (which I initially read as 'Can't locate atv stream!'), then find the IPlayer plugin icon in your Videos section, bring up the context menu (the title key on my remote shows this) and go to Plugin Settings. Change the video stream from 'Auto' to H.264 (1500kb) or H.264(800kb) depending on how fast your connection is - try the high quality stream first.
I don't know exactly why the Auto stream doesn't work anymore, so this is really just a workaround, but it does seem to work for me and others, so that's good enough for me!
Hope this helps.